__construct的可见性(Visibility of __construct)

 如果我有： 
 
abstract class AbstractSingleton
{
    protected static $instance;

    public static function & getInstance()
    {
        if(null === static::$instance) static::$instance = new static();
        return static::$instance;
    }

    protected function __construct() { }
}
 
 扩展该类的用户是否可以重新定义__construct的可见性？ 
 
class Singleton extends AbstractSingleton
{
    public function __construct() { } // That would be a problem
}
 
 第二个问题，如果在AbstractSingleton中__construct被定义为私有，那么子类会发生什么？ 有没有构造函数？ 是否为该类提供了一个新的默认构造函数，如果是，具有何种可见性？ 

If I have:
 
abstract class AbstractSingleton
{
    protected static $instance;

    public static function & getInstance()
    {
        if(null === static::$instance) static::$instance = new static();
        return static::$instance;
    }

    protected function __construct() { }
}
 
Can a user who extends the class redefine the visibility of __construct?
 
class Singleton extends AbstractSingleton
{
    public function __construct() { } // That would be a problem
}
 
And second question, what happens for subclasses if __construct is defined as private in AbstractSingleton? Is there not constructor at all? Is a new default constructor provided to the class, and if yes, with what visibility?

原文：https://stackoverflow.com/questions/13711764